For #18198 - Add a feature flag for Credit Cards and Addresses autofill feature

[gabrielluong]

Fixes #18198

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

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[jonalmeida]

LGTM ✨

[mergify]

This pull request has conflicts when rebasing. Could you fix it @gabrielluong? 🙏

[codecov-io]

Codecov Report

Merging #18200 (d40902c) into master (607d638) will increase coverage by 0.13%.
The diff coverage is 55.55%.

@@             Coverage Diff              @@
##             master   #18200      +/-   ##
============================================
+ Coverage     32.96%   33.09%   +0.13%     
- Complexity     1413     1415       +2     
============================================
  Files           460      460              
  Lines         19440    19458      +18     
  Branches       2987     2987              
============================================
+ Hits           6408     6440      +32     
+ Misses        12377    12363      -14     
  Partials        655      655              

Impacted Files	Coverage Δ	Complexity Δ
...g/mozilla/fenix/settings/SecretSettingsFragment.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...pp/src/main/java/org/mozilla/fenix/FeatureFlags.kt	100.00% <100.00%> (ø)	6.00 <2.00> (+2.00)
.../src/main/java/org/mozilla/fenix/utils/Settings.kt	72.54% <100.00%> (+0.45%)	36.00 <0.00> (ø)
...rg/mozilla/fenix/settings/account/AccountUiView.kt	32.50% <0.00%> (-2.50%)	4.00% <0.00%> (ø%)
...components/toolbar/BrowserToolbarMenuController.kt	70.15% <0.00%> (+9.94%)	0.00% <0.00%> (ø%)
...rg/mozilla/fenix/components/toolbar/ToolbarMenu.kt	86.95% <0.00%> (+17.39%)	0.00% <0.00%> (ø%)

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